"World War Three" is the fifth episode of the first series of the British science fiction television programme Doctor Who which was first broadcast on BBC One on 23 April 2005. It is the second of a two-part story which began with "Aliens of London" on 16 April.

In the episode, set in London, the alien time traveller the Ninth Doctor (Christopher Eccleston) and his companion Rose Tyler (Billie Piper) team up with Rose's boyfriend Mickey Smith (Noel Clarke), her mother Jackie (Camille Coduri), and Member of Parliament Harriet Jones (Penelope Wilton) to foil the plan of the alien Slitheen family from selling the Earth for commercial purposes. The Slitheen, who have infiltrated the Government of the United Kingdom, plan to get the United Nations to release nuclear activation codes so they can trigger World War III on Earth and sell the remains.